Add AppImage build instructions
This PR add instructions for producing an AppImage on each build. It's built using appiamge-builder and can be used in almost every modern GNU/Linux distribution. It require some additional tuning from your side in order to properly set the version.
Please review and let me know your comments.
Providing an AppImage would have, among others, these advantages:
Applications packaged as an AppImage can run on many distributions (including Ubuntu, Fedora, openSUSE, CentOS, elementaryOS, Linux Mint, and others) One app = one file = super simple for users: just download one AppImage file, make it executable, and run No unpacking or installation necessary No root needed No system libraries changed Works out of the box, no installation of runtimes needed Optional desktop integration with appimaged Optional binary delta updates, e.g., for continuous builds (only download the binary diff) using AppImageUpdate Can optionally GPG2-sign your AppImages (inside the file) Works on Live ISOs Can use the same AppImages when dual-booting multiple distributions Can be listed in the AppImageHub central directory of available AppImages Can double as a self-extracting compressed archive with the --appimage-extract parameter No repositories needed. Suitable/optimized for air-gapped (offline) machines Decentralized